Method and apparatus for detecting an analyte

ABSTRACT

A method for determining a concentration of an analyte is provided. The method includes providing a substrate including a conductive region and a recognition layer where the conductive region has a first surface operatively coupled with the recognition layer; The method also includes contacting the substrate with the sample to bind at least some of the analyte that may present in the sample with the recognition layer; The method further includes directing radiation through the conductive region and the recognition layer, where the conductive region comprises at least one particle and a combination of the at least one particle effect when the radiation is directed through the conductive region and the recognition layer; The method still further includes measuring at least a part of a spectrum of the radiation that is absorbed or transmitted by or through the substrate, the at least part of the spectrum being related to one or more of the at least one particle effects; The method further includes determining a change of the at least part of the spectrum as compared with a reference spectrum and determining the concentration of the analyte from the change.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority as aContinuation-in-Part of U.S. patent application Ser. No. 10/322,901,filed Dec. 18, 2002 now abandoned, which claims the benefit of priorityof U.S. Provisional Patent Application Ser. No. 60/345,169, filed Dec.21, 2001. This application also claims priority benefits as aContinuation-in-Part Application of U.S. patent application Ser. No.11/175,729, filed on Jul. 5, 2005, which claims the benefit of priorityof U.S. Provisional Patent Application Ser. No. 60/584,953, filed Jul.2, 2004. The present application also claims the benefit of priority ofU.S. Provisional Patent Application 60/733,277, filed Nov. 2, 2005.

The disclosures of U.S. patent application Ser. No. 10/322,901, U.S.Provisional Patent Application 60/345,169, U.S. patent application Ser.No. 11/175,729, U.S. Provisional Patent Application 60/584,953, and U.S.Provisional Patent Application 60/733,277 are incorporated by referenceherein in their entirety.

BACKGROUND

I. Field of the Invention

This disclosure relates generally to assaying and, more specifically, tomethods and apparatus for assaying an analyte.

II. Description of Related Art

Different types of biosensors are known, along with their specificadvantages and disadvantages. For example, electrochemical biosensors,surface acoustic wave sensors and surface plasmon resonance biosensorsare known biosensors that have the advantage of not requiring the use oflabeling techniques for most applications. However, these sensors alsohave certain disadvantages.

A current method for assaying an analyte (e.g., antibodies and/orantigens) in a fluid sample is accomplished using surface plasmonresonance (“SPR”) techniques. In such a technique, the presence of ananalyte is determined by a change in the refractive index at a solidoptical surface when the analyte interacts with a refractive indexenhancing species, thereby causing binding or release of the speciesfrom the surface. In this respect, the SPR signal is a measure of massconcentration at a sensor chip surface. This means that theanalyte/ligand association and dissociation in the sample can beobserved and, ultimately, rate constants as well as equilibriumconstants can be determined.

However, as was noted above, acquiring such SPR measurements has somedisadvantages. In this respect, systems for carrying out such analysesare typically expensive as those systems generally employ a quartz prismas well as a radiation source that is capable of generating polarizedlight. Also, the response of an SPR sensor depends on the volume andrefractive index of the bound analyte. Therefore, for very smallmolecules, this technique results in very small changes of refractiveindex and can make detecting such analytes difficult.

Another current approach for detecting an analyte is the use of anoptical based sensing device for detecting the presence (and amount of)an analyte using both indicator and reference channels. The sensingdevice used in such an approach typically includes a sensor body with aradiation source contained therein. The radiation emitted by the sourceinteracts with molecules in the material (i.e., the sample) beinganalyzed, which typically results in a change of at least one opticalcharacteristic of those molecules. Such a change is not desirable as itmay reduce the accuracy of information regarding the analyte obtained.Therefore, based on the foregoing, a need exists for improved methodsand apparatus for assaying analytes.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY OF THE INVENTION

The following embodiments and aspects thereof are described andillustrated in conjunction with devices and methods which are given byway of example and meant to be illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In a first aspect, the invention comprises a method for determining theconcentration of an analyte within a sample, the method comprising:

providing a first substrate comprising a conductive region and arecognition layer, the conductive region having a first surfaceoperatively coupled (i.e., in close proximity, in which the conductiveregion and recognition layer may be, for example, covalently coupled,coupled via linking molecules, or the recognition layer adsorbed on theconductive region) with the recognition layer, the recognition layercomprising at least one recognition molecule,

wherein said conductive region comprises at least one particle andwherein the distance between the first surface of the conductive regionand the recognition molecule is selected such that when the analyte isbound to the recognition layer the combination of the at least oneparticle and the analyte exhibits at least one of the following effectswhen radiation is directed through the conductive region and therecognition layer: (i) a particle plasmon effect, (ii) a particle bulkinterband absorption, (iii) analyte molecular absorption, and (iv)absorption by the analyte-particle combination;

contacting the substrate with the sample to bind at least some of anyanalyte present in the sample with the recognition molecule;

directing radiation through the conductive region and the recognitionlayer,

measuring at least a part of a spectrum of radiation that is absorbed ortransmitted by or through the substrate, the at least part of thespectrum manifesting at least one of (i) a particle plasmon effect, (ii)a particle bulk interband absorption, (iii) an analyte molecularabsorption and (iv) an absorption by the analyte-particle combination;

comparing the at least part of the spectrum with a reference spectrumand determining the difference; and

determining the concentration of the analyte from the difference.

In a preferred embodiment of the first aspect of the invention, thedistance between the first surface and the recognition molecule is lessthan 60 nm.

The distance between the first surface of the conductive region and therecognition molecules is preferably selected such that it is less than60 nm, less than 50 nm, less than 40 nm, less than 30 nm or less than 25nm, less than 20 nm, less than 15 nm or even less than 10 nm. In oneembodiment the distance is less than 17 nm, preferably between 4 and 17nm. The distance between the recognition molecule and the first surfaceof the conductive region is selected such that at least one of effects(i) to (iv) is observed.

This allows more sensitive measurements because the transmission oflight through the conductive region creates a surface plasmon resonancewave, the length of this wave decaying exponentially away from theconductive region. The surface plasmon resonance wave results in theeffects (i) to (iv). The distance is measured from the first surface upto the part of the recognition molecule where the binding with theanalyte takes place.

In a particular embodiment of the first aspect, the distance between therecognition molecule and the first surface is obtained by selecting theappropriate size of the recognition molecules. The recognition moleculewill be positioned close to the first surface of the conductive region,meaning at a distance at least less than 60 nm, less than 50 nm, lessthan 40 nm, less than 30 nm, less than 25 nm less than 20 nm, less than15 nm or even less than 10 nm from the first surface of the conductiveregion. The distance should be between 1 nm and 60 nm, between 1 nm and50 nm, between 1 nm and 40 nm, between 1 nm and 30 nm, between 1 nm and20 nm, between 1 nm and 10 nm, 5 nm and 40 nm, between 5 nm and 35 nm orbetween 5 nm and 25 nm. The recognition molecule can have a molecularweight less than 150 000 Dalton, less than 100 000 Dalton, less than 80000 Dalton, less than 70 000 Dalton, less than 60 000 Dalton, less than50 000 Dalton, less than 40 000 Dalton, less than 30 000 Dalton, lessthan 20 000 Dalton. In a preferred embodiment, the molecular weight isbetween 200 Dalton and 40 000 Dalton or even between 10 000 Dalton and40 000 Dalton.

The recognition molecule can be a small molecule such as a hormone,peptide, or antibiotic. In other cases the recognition molecule cantreated such that only the active part (the part that binds the analyte)of the recognition molecule is part of the recognition layer. Inparticular, the recognition molecule can be subjected to cleavage withenzymes such as proteases or chemical reducing agents such asdithiotreitol (Fab, Fab′, (Fab′)₂). The recognition molecule can be asingle chain Fv fragment (ScFv). In another embodiment, the recognitionmolecule can be a recombinant camel antibody fragment (VHH).

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments of this invention isdisclosed wherein the at least one particle exhibits a particle plasmoneffect and a bulk interband absorption and a plasmon coupling band whenanalyte is bound to the recognition molecule.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the at least oneparticle is formed of a material selected from the group consisting ofdiamond, a metal, a semiconductive material and combinations thereof.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the at least oneparticle is an alloy of at least two metals.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the at least oneparticle is formed of a semiconductive material.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the at least oneparticle is formed of a core material and a shell material.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the conductiveregion comprises semiconductive particles and metallic particles.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the conductiveregion comprises at least two particles and the edge to edge distance ofthe at least two particles is between 1 nm and 5 μm, preferably between1 nm and 1 μm.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the conductiveregion is arranged such that when radiation is transmitted through thesubstrate, measuring an intensity of the radiation absorbed ortransmitted by or through the substrate is performed in a wavelengthregion between 200 nm and 1200 nm.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the averagediameter of the at least one particle is smaller in dimension than aprincipal wavelength of the radiation.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the averagediameter of the at least one particle is less than 300 nm.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein an interactionbetween the analyte and the recognition layer results in a change in adielectric constant of the recognition layer.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the substratefurther comprises a support layer and a second surface of the conductiveregion, the second surface being operatively coupled with the supportlayer.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the support layeris optically transparent to the radiation. In such embodiments, onemeasures the radiation that is transmitted through the layer and thesubstrate, and, therefore, the substrate is (semi-)transparent for thewavelength that is used. In another embodiment of the first aspect, theinvention comprises a method as recited in any of the previousembodiments the substrate is a least partially reflective, and onemeasures the reflectance.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the support layeris optically transparent or semi-transparent to the radiation. By“semi-transparent” is meant that less than 100% transparent but thatenough light can be transmitted through the substrate to be able tomeasure the transmission.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the recognitionlayer comprises an intermediate layer (between 0.5 nm and 3 nm thick)and a recognition molecule. The recognition molecule may be associatedwith the intermediate layer in any manner that immobilizes therecognition molecule in, on, or connected to the intermediate layer.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the recognitionlayer comprises a self-assembling monolayer as an intermediate layer.

In general, any layer that can immobilize the analyte, thereby showing aplasmon effect, can be used. A self-assembling monolayer is but oneexample of a layer that “operatively” can couple the recognitionmolecule to the conductive region. Self-assembling monolayers providethe following advantages:

-   -   1) Self-assembled monolayers (“SAMs”) with reactive groups such        as thiols can attach covalently via the reactive (e.g., thiol        group) to the conductive region (e.g., to nobel metal        nanoparticles) and are thus stable;    -   2) SAMs generally have limited thickness, making it possible to        detect closer to the conductive region (e.g., nanoparticle)        surface;    -   3) SAMs allow the integration of chemical groups that can be        used to covalently attach the biomolecules onto the conductive        region surface (e.g., nanoparticle film) and enable control of        binding events, such as bind specificity, thereby making        detection reproducible.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the substratecomprises a plurality of conductive regions, the plurality of conductiveregions being arranged in an array.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the substrate isarranged as a microtitre plate.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein an intensity ofthe radiation absorbed or transmitted by or through the substrate isdetermined as a function of a wavelength of the radiation.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein instead ofcomparing at least a part of a spectrum of radiation that is absorbed ortransmitted by or through the substrate with a reference spectrum, themethod comprises:

providing a second substrate, which comprises a conductive region and norecognition layer or a recognition layer that comprises molecules havingno analyte binding capacity or non-specific binding capacity;

subjecting the second substrate to an analyte-containing referencesample

directing radiation through the second substrate;

measuring the intensity of the radiation absorbed or transmitted by orthrough the second substrate; and

comparing an intensity of the radiation absorbed or transmitted by orthrough the second substrate with an intensity of the radiation absorbedor transmitted by or through the first substrate to determine theconcentration of the analyte on the first substrate.

In this embodiment, the signal observed with the second substrate can besubtracted from the signal of the sample with specific receptormolecules for the analyte of interest. A third substrate can also beused in conjunction with the second substrate, wherein the thirdsubstrate comprises the same recognition molecules as in the firstsubstrate, the third substrate is subjected to a sample without analyte,and the transmission/adsorption is measured and subtracted from themeasurements of the first substrate.

In an embodiment of the first aspect, the invention comprises a methodas recited in any of the previous embodiments wherein the conductiveregion comprises at least two particles and a combination of the atleast two particles and the analyte further exhibits a plasmon couplingeffect.

An example method for analyzing and determining the presence of ananalyte within a sample is disclosed. In one embodiment, such a methodincludes providing a substrate having a conductive region and arecognition layer. The conductive region of the substrate has at leasttwo surfaces, a first surface and a second surface, wherein the firstsurface is operatively associated with the recognition layer. Therecognition layer of the substrate is contacted with the analyte so thata reaction occurs between the analyte and the recognition layer. As usedherein, a “reaction” is any binding event which immobilizes the analyteon, near, or to the recognition layer. The recognition layer comprisesat least one recognition molecule, which can bind with the analyte.After this reaction, radiation is directed through the conductive regionand the recognition layer. By measuring the intensity of the radiationabsorbed and/or transmitted by the substrate as a function of theradiation's wavelength, the presence of an analyte can be determined.The method also can be used generally for affinity immunosensing andbiosensing. For instance, by optically monitoring the recognition layer,one may detect the presence of antigens captured by surface immobilizedantibodies.

Further in this embodiment, the conductive region includes at least oneparticle, which has an average diameter that is preferably smaller thanthe wavelength of the impinging radiation. The interaction between theanalyte and the conductive region affects the dielectric constant of theconductive region and the recognition layer, which results in a changein the absorption or transmittance spectrum of the substrate. Moreover,this interaction results in a change in plasmon resonance frequency,which is mainly determined by the dielectric function of the conductiveregion and the surrounding medium. For this arrangement, the surroundingmedium includes the recognition layer and the conductive particle(s).The recognition layer may be, for example, a self-assembling monolayer.FIG. 26 displays an example of a mixed SAM, but other suitable SAM's maybe used as well. Other examples include polymers and silanes that can beused to immobilize the receptor molecules on the substrate.

In certain embodiments, the average diameter of the conductiveparticle(s) is (are), alternatively less than 300 nm, less than 200 nm,less than 100 nm or less than 50 nm, depending on the particularapplication and the specific analyte being assayed. The conductiveregion may include a metal on which a plasmon effect can be induced. Forexample, such a metal may be selected from the group consisting of gold,silver and copper.

In other embodiments, the substrate also further includes a supportlayer. In this arrangement, a second surface of the substrate isoperatively coupled with the support layer, where the support layer isoptically transparent or semi-transparent.

In still other embodiments, the substrate may have multiple conductiveregions, which are ordered in an array arrangement. For instance, thesubstrate may be a microtitre plate that is constructed so as to be usedfor high-throughput screening.

In another embodiment, a method for analyzing and determining thepresence of an analyte within a sample includes providing a secondsubstrate, where the second substrate includes a conductive regionhaving at least a first surface and a second surface and a recognitionlayer, where a first surface of the conductive region is operativelycoupled with the recognition layer. The method further includessubjecting the second substrate to a reference sample, then directingradiation through the conductive region and the recognition layer of thesecond substrate. The method still further includes measuring theintensity of the radiation absorbed and/or transmitted by the secondsubstrate as a function of wavelength and comparing that intensity withthe intensity of the radiation absorbed or transmitted by the firstsubstrate. Using this comparison, the presence of an analyte can bedetermined.

Such techniques provide for assaying an analyte without labeling, wheresuch techniques are relatively sensitive, as compared with priortechniques. Furthermore, such techniques are less complex and have lowercost than current approaches. Such techniques may be used, for example,for assaying biomolecules.

In another aspect, the invention comprises a substrate as describedabove in any of the embodiments of the methods described above. In apreferred embodiment, in the substrate described hereinabove, thedistance between the first surface of the conductive region and therecognition molecules is preferably selected such that it is less than60 nm, less than 50 nm, less than 40 nm, less than 30 nm or less than 25nm, less than 20 nm, less than 15 nm or even less than 10 nm. Thedistance between the recognition molecule and the first surface of theconductive region is selected such that upon binding of an analyte tothe recognition molecule at least one (i) a particle plasmon effect,(ii) a particle bulk interband absorption, (iii) analyte molecularabsorption, and (iv) absorption by the analyte-particle combination maybe observed.

In this aspect, the invention comprises a substrate for determining theconcentration of an analyte within a sample, the substrate comprising:

-   -   a conductive region and a recognition layer, the conductive        region having a first surface operatively coupled with the        recognition layer, the recognition layer comprising at least one        recognition molecule,    -   wherein said conductive region comprises at least one particle        and wherein the distance between the first surface of the        conductive region and the recognition molecule is selected such        that when the analyte is bound to the recognition layer the        combination of the at least one particle and the analyte        exhibits at least one of the following effects when radiation is        directed through the conductive region and the recognition        layer: (i) a particle plasmon effect, (ii) a particle bulk        interband absorption, (iii) analyte molecular absorption,        and (iv) absorption by the analyte-particle combination;    -   wherein the distance between said first surface and the part of        said recognition molecule where binding takes place is less than        60 nm.

In preferred embodiments of this aspect of the invention:

-   a) the distance between said first surface and the part of said    recognition molecule where binding takes place is less than 17 nm;-   b) the distance between said first surface and the part of said    recognition molecule where binding takes place is between 4 and 17    nm;-   c) the recognition molecule is subjected enzymatic cleavage such    that only the active part of the recognition molecules is part of    the recognition layer;-   d) the recognition molecule is a small molecule that functions as a    recognition element in an inhibition or replacement assay;-   e) the at least one particle exhibits a particle plasmon effect and    a bulk interband absorption and a plasmon coupling band;-   f) the at least one particle is formed of a material selected from    the group consisting of diamond, a metal, a semiconductive material    and combinations thereof;-   g) the at least one particle is an alloy of at least two metals;-   h) the at least one particle is formed of a semiconductive material;-   i) the at least one particle is formed of a core material and a    shell material;-   j) the conductive region comprises semiconductive particles and    metallic particles;-   k) the conductive region comprises at least two particles and the    edge to edge distance of the at least two particles is between 1 nm    and 5 μm;-   l) the conductive region comprises at least two particles and the    edge to edge distance of the particles is between 1 nm and 1 μm;-   m) the conductive region is arranged such that when radiation is    transmitted through the substrate, measuring an intensity of the    radiation absorbed or transmitted by or through the substrate is    performed in a wavelength region between 200 nm and 1200 nm;-   n) the average diameter of the at least one particle is smaller in    dimension than a principal wavelength of the radiation;-   o) the average diameter of the at least one particle is less than    300 nm;-   p) an interaction between the analyte and the recognition layer    results in a change in a dielectric constant of the recognition    layer;-   q) the substrate further comprises a support layer and a second    surface of the conductive region, the second surface being    operatively coupled with the support layer, preferably wherein (i)    the support layer is optically transparent to the radiation or (ii)    the support layer is optically semi-transparent to the radiation;-   r) the recognition layer comprises an intermediate layer and a    recognition molecule;-   s) the recognition layer comprises a self-assembling monolayer;-   t) the substrate comprises a plurality of conductive regions, the    plurality of conductive regions being arranged in an array;-   u) the substrate is arranged as a microtitre plate;-   v) the conductive region comprises at least two particles and a    combination of the at least two particles and the analyte further    exhibits a plasmon coupling effect.

In another aspect, the invention comprises an apparatus/device forconducting the method as described hereinabove. Such an apparatuscomprises a light source for illuminating a substrate, a substrate asdescribed hereinabove, and a light detection device for detecting theintensity of radiation from the light source absorbed or transmitted byor through the substrate.

We have found that the plasmon resonance signal is higher/better/morereliable when the distance between the particles and the conductingsubstrate is smaller, as described here and above. Having a smallerdistance enhances sensitivity and permits detection and quantificationof smaller analyte concentrations.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1 is a drawing illustrating an experimental apparatus for detectingan analyte;

FIG. 2 is drawing illustrating a schematic representation of aconventional ELISA method of antigen detection;

FIG. 3 is a drawing illustrating a schematic representation of slidesand quartz cells that may be used to implement methods for detecting ananalyte;

FIG. 4 a is a graph illustrating absorbance spectra for (i) a thin goldfilm on quartz, (ii) deposited Human Serum Albumin (“HSA”) on the thingold film on quartz; and (iii) deposited HSA with subsequently depositedanti-HSA on the thin gold film on quartz;

FIG. 4 b is a graph illustrating difference spectra of the spectra ofFIG. 4 a (HSA and HSA+anti-HSA), which are background corrected for theabsorbance spectrum of the thin gold film on quartz;

FIG. 5 a is a graph illustrating absorbance spectra generated for (i)thin gold film; (ii) self-assembled monolayers of thiols on the thingold film; and (iii) HSA at different concentrations and differentdeposition times on the thin gold film;

FIG. 5 b is a graph illustrating difference spectra of the spectra ofFIG. 5 a (thiols and various HSA depositions), which are backgroundcorrected for the absorbance spectrum of the thin gold film;

FIG. 6 is a graph illustrating difference spectra for variousimmunosensing applications in accordance with the invention;

FIG. 7 is a graph illustrating absorbance spectra of Au particles insolution and on a surface;

FIG. 8 is a drawing illustrating a schematic representation of a methodfor forming a conductive region and a recognition layer;

FIG. 9 is a drawing illustrating a top view of a nanoparticle film;

FIG. 10 is a drawing illustrating a schematic representation of a methodfor forming a substrate using a “polystyrene nanoparticle template”method;

FIG. 11 is drawing illustrating a schematic representation of a sandwichassay;

FIG. 12 is a drawing illustrating a schematic representation of acompetition assay.

FIG. 13 is a drawing illustrating a sandwich assay apparatus.

FIG. 14: Schematic drawing of an antibody-antigen interaction on ananoparticle film, covered with self-assembled monolayers of thiols,which bind antibodies, which can bind antigen.

FIG. 15: Enzymatic cutting of an antibody to F(ab′)₂ using pepsin.

FIG. 16: Chemical reduction of a F(ab′)₂ fragment to two fully activeFab′ fragments.

FIG. 17 Top: Conventional antibody structure and its fragments (Fab, Fv,ScFv). Bottom: Heavy Chain antibody and its VHH fragment

FIG. 18: Schematic representation of the performed experiment concerningPSA detection using VHH fragments

FIG. 19: The detection of different concentrations of PSA measured afterdifferent time frames using the differential transmission plasmonbiosensing measurements. The white bars indicate the results using camelantibody fragments as bioreceptors, while the black bars indicate someresults realized using normal mouse monoclonal antibodies.

FIG. 20: A schematic illustration of a TPB sandwich approach. Frombottom to top: quartz substrate, mercaptosilane adhesion layer, Au 50 nmgold nanoparticles, mixed self-assembled monolayer, camel antibodyfragment (VHH), PSA, secondary anti-PSA antibody functionalized with 20nm gold nanoparticles

FIG. 21: The detection of 10 ng/mL of PSA via a direct measurement orvia an sandwich approach using gold labeled secondary antibodies.

FIG. 22: (a) penicillin specific antibodies are bound to the surfacemodified with the analyte analogue (ampicillin). (b) the surface boundantibodies are replaced by the analyte molecules (ampicillin) in thesample.

FIG. 23: Changes of the absorption spectra during binding of theanti-penicillin antibodies (clone 19C9) to the ampicillin modifiednanoparticle film in function of the time.

FIG. 24: Changes of the maximum absorption during binding of theanti-penicillin antibodies (clone 19C9) to the ampicillin modifiednanoparticle film in function of the time and this in the presence ofdifferent concentrations of ampicillin (100 ng/ml-100 μg/ml).

FIG. 25: Schematic presentation of the different immunoassay formatsthat can be used a) the direct assay, b) the sandwich assay, c) theindirect competition assay and d) the indirect inhibition assay.

FIG. 26: Mixed self-assembled monolayer to functionalize the surface ofthe gold nanoparticles.

FIGS. 27(A)-(E): Schematic representations of various configurations ofthe conductive region.

DETAILED DESCRIPTION

Overview

The methods of the invention for detecting an analyte can be used, forexample, in conjunction with sensing devices that have a highsensitivity and that can be used to detect very low concentrations ofcertain analytes. In particular, such a method may be used for affinityimmunosensing and biosensing. Such a method may include opticallymonitoring the deposition of an intermediate layer or linking layer (theterms “intermediate layer” and “linking layer” are used interchangeablyherein), recognition of an immobilized recognition molecule such asantibody, antigen, enzyme, cells hormone, peptides, receptor moleculesand nucleic acid, and recognition of the analyte, which involvesoptically monitoring for recognition of the analyte (e.g., antigen) byan immobilized receptor molecule (e.g., antibody). The method isversatile, and allows for applications in both the liquid and gasphases, while also allowing for quantitative in situ measurements.

More specifically, a method for assaying an analyte in a sample (e.g., aliquid sample) may be accomplished by bringing the sample into physicalcontact with a substrate, where the substrate includes a conductiveregion and a recognition layer. The presence of the analyte (in thesample) being assayed is then determined by a resulting change in thespectrum (wavelength/frequency vs. intensity) of radiation transmittedthrough the substrate (or absorbed by the substrate) resulting from thepresence (or non-presence) of the analyte (e.g., as compared with areference spectrum where no analyte is present).

For purposes of this disclosure, it should be understood that the terms“absorbed radiation (or absorbance)” and “transmitted radiation (ortransmittance)” can be used interchangeably. In this respect, therelationship between the absorbance (A) and the transmittance (T) may begiven by the equation: A=−log T.

Exemplary Apparatus for Assaying an Analyte

Referring now to FIG. 1, a drawing illustrating an exemplary embodimentof an experimental apparatus 10 for assaying an analyte in a sample isshown. The apparatus 10 includes a substrate 11. The substrate 11 has aconductive region 12 and a recognition layer 13. The conductive region12 has a first surface 14 and a second surface 15, where the firstsurface 14 is operatively coupled with the recognition layer 13.

When the substrate 11 is subjected to (e.g., placed in physical contactwith) a sample containing the analyte (not shown), an interaction (ifthe analyte is present) occurs between the analyte and the recognitionlayer 13. After such an interaction has occurred, radiation 16 isdirected through the substrate 11, such that the radiation 16 isincident with the conductive region 12 and the recognition layer 13. Thepresence and/or concentration of the analyte in the sample is thendetermined by measuring the intensity (or amount) of the radiation 16that is absorbed by (or transmitted through) the substrate 11. Thismeasurement is a function of the wavelength of radiation 17 (e.g., theradiation that is communicated through the substrate 11).

As shown in FIG. 1, the substrate 11, for this embodiment, furthercomprises a support layer 18, where the second surface 15 of theconductive region 12 is operatively coupled with the support layer 18using a molecular glue of for example a SAM of mercapto-silanes; Thesupport layer 18 provides mechanical support to the substrate 11 andfacilitates subjecting the substrate 11 to samples being assayed usingthe substrate 11; By “operatively coupled” we mean that the secondsurface of the conductive region and the support layer are in closeproximity (e.g., covalently linked, one adsorbed on another, coupled vialinking molecules, etc.).

The radiation 16 shown in FIG. 1 may be generated by any number ofradiation sources such as (without limitation) an incandescent lamp, alight emitting diode (LED) or a laser. In the exemplary embodiment, aradiation source 19 for producing the radiation 16 preferably suppliesthe radiation 16 with a principal wavelength in the range correspondingto a maximum absorbance (or a minimum transmittance) wavelength of thesubstrate. The maximum absorbance of the substrate or the maximumabsorbance of the combination of substrate with analyte adsorbed theretoboth can be used. Often only the maximum absorbance of the substrate isknown, however. In that case only the maximum absorbance of thesubstrate can be used. If known, it may be preferable to use the maximumabsorbance of the combination of the substrate with the analyte adsorbedthereto. For instance, the radiation source 19 may provide the radiation16 with a principal wavelength between 200 nm and 1500 nm.

For the apparatus 10, the radiation source 19 can take the form of alight emitting diode (“LED”), such as a red LED or a blue LED. Ofcourse, other sources of radiation may be used. Alternatively, forexample, the radiation source 19 may take the form of a focused beamsource (e.g., a laser) or may be a source that provides a broaderspectrum of light. Also, in certain embodiments, it may be advantageousfor the radiation source 19 to provide collimated radiation, which maybe either monochromatic or polychromatic radiation.

Furthermore, the apparatus 10 may include additional components (whichare not shown) such as one or more devices placed between the radiationsource 19 and the substrate 11, such as to collimate the radiation 16.These devices include, but are not limited to optical lenses, slottedgates, and grated filters. Additionally, a radiation sensor is typicallyused for analyzing the radiation 17 that is communicated through thesubstrate 11. Such sensors include (also without limitation)commercially available UV-Vis spectrometers. Alternatively, such asensor may take the form of an absorptiometers or a calorimeter.

As previously discussed, the apparatus 10 shown in FIG. 1 includes thesupport layer 18, which provides mechanical support to the substrate 11.The support layer 18 is typically optically transparent orsemi-transparent with regard to the principal wavelength of theradiation 16 provided by the radiation source 19. For example, thesupport layer 18 should have an optical transparency between 5% and 95%,with a transparency between 20% and 80% being preferable in certainapplications. To provide this optical transparency, the support layer 18may be formed using materials such as glass, quartz, or a polymericmaterial (such as polycarbonate, polysulphonate andpolymethyl-methacrylate).

Depending on the material used to form the support layer 18, the supportlayer 18 may be flat. Alternatively, the support layer 18 may be part ofa glass or quartz tube, a polymeric tube or a microtitre plate, whichmay be integrated in a flow system. The substrate 11 may also be amicrotitre plate that is part of a high-throughput screening system oran apparatus for performing ELISA assay methods.

In an example embodiment, the conductive region 12 is formed of amaterial which exhibits one or more of (i) a particle (colloidal)plasmon effect, (ii) a bulk interband absorption effect and (iii) aplasmon coupling band effect. As was noted above, the first surface 14of the conductive region 12 is operatively coupled with the recognitionlayer 12. Also, as shown in FIG. 1, the second surface 15 of theconductive region 12 is in contact with an external medium, in this casethe support layer 18. Alternatively, the second surface 15 may be inair, or another gas.

In order to achieve one or more of the above effects, the particles ofthe conductive region 12 of substrate 11 may be formed of a conductivematerial, such as a conductive metal. In this respect, conductive region12 may be formed from, for example, particles of gold, silver, orcopper. However, it will be appreciated that the conductive region 12may be formed from particles of any conductive material on which aplasmon effect can be induced. For example, the conductive region 12,alternatively, may be formed from particles of a conductive glass,conductive polymers or metallic nanoparticles. The conductive region 12may also be formed of particles of diamond, a metallic material, asemi-conductive material or combinations thereof. As was noted above,the conductive region 12 includes at least one particle of the materialselected for forming the conductive region 12.

Depending on the particular embodiment, the conductive region 12 mayalso be formed from particles of an alloy of at least two metals, whichmay be selected from the alloys Ag/Au, Si/Au, Si/Ag, Cu/Au, though otheralloys are possible. In other embodiments, the conductive region 12 maybe formed from particles of a combination of metallic materials andsemi-conductive materials.

In other embodiments, the conductive region 12 may be formed fromparticles of (i) a semi-conductive material, (ii) a nanoshell or (iii) acombination of semi-conductive particles with metal particles and/or oneor more metal alloy materials. For embodiment using semiconductivematerials, such material may be selected such that conductive regiondemonstrates absorption characteristics from the low UV region to theVIS region to the near-IR region. Such materials include CdS, CdSe, PbS,PbSe, ZnS, CdTe, ZnSe, ZnTe, HgSe, HgS, HgTe, PbTe, and GaN, thoughother materials may also be appropriate in such an embodiment.

For embodiments using a nanoshell, the conductive region 12 may includeparticles of a core material and a shell material. Some example materialcombinations that may used to implement such nanoshells are (i) CdSe(core) and ZnS (shell); (ii) CdS (core) and ZnS (shell); (iii) CdSe(core) and Au (shell); (iv) CdSe (core) and Ag (shell); (v) CdS (core)and Au (shell); (vi) CdS (core) and Ag (shell); and (vii) ZnSe (core)and Au (shell). Of course, other material combinations are possible.

The conductive region 12 typically has a thickness of less than 60 nm,less than 50 nm, less than 40 nm, less than 30 nm, less than 20 nm, orless than 10 nm, with a thickness of less than 5 nm being preferable.Further in this regard, while the conductive region 12 is shown as beingcontinuous in FIG. 1, alternatively the conductive region may bediscontinuous, such that islands of conductive material are formed onthe support layer 18, these islands having either uniform or non-uniformthickness.

With reference to FIGS. 27(A)-(E), in another embodiment the conductiveregion may comprise a support layer and a conductive layer comprisedparticles (as described herein), wherein the support layer is asemi-transparent or transparent substrate on/in which the conductivematerial is formed. The support layer can be conductive ornon-conductive. The particles can be deposited on top of the substrateor can be incorporated in the substrate. The conductive material may beparticles that are not connected, so form a discontinuous layer. Theconductive layer may also be particles that are partially connected. Theconductive layer may also be particles that are fully connected, but notfully covering the substrate. The conductive layer may also be particlesthat are connected and fully cover the substrate or support layer. Theconductive layer may also be a uniform layer of conductive material. Theparticles can be randomly distributed over the substrate or can bearranged in a geometrical array. The particles may be homogenous orheterogenous in size. If heterogenous, the size may show a distributionsuch as Gaussian. The particles may be made of one material or acombination of particles of different materials can be used. In fact,any material that can manifest a surface Plasmon effect can be used.Also, polymer particles embedded in a conductive material (for exampleAu) showing surface Plasmon effect can be used.

The conductive material can be deposited by any deposition method knownin the art, such as sputter deposition, PVD CVD, plating. Particles canbe formed by evaporating a thin non-continuous layer. Particles can alsobe put on the substrate. Particles can also be made by evaporating aconducting layer on the substrate and consequently patterning in awell-defined pattern thereby creating isolated islands of the conductingmaterial.

As an alternative to using a single particle to form the conductiveregion 12, the conductive region 12 may be formed of a plurality ofparticles. In such an embodiment, the conductive region 12 may be formedusing a plurality of particles that are spaced in such a way that theparticles exhibit an appropriate surface plasmon absorption spectrum foruse in detecting the presence and/or concentration of an analyte. Theedge-to-edge distance of such particles (e.g., the inter-particledistance) on the surface of the conductive region 12 can vary from 1 nmto several μm. An inter-particle spacing of 5 to 500 nm is typical forcertain materials.

In the apparatus 10 shown in FIG. 1, the conductive region 12, aspreviously described, typically includes conductive particles and, moreparticularly, microparticles or nanoparticles, as such particles aresmaller in molecular size than the preferable wavelength of theradiation 16 that impinges on the particles (and the substrate 11). Theaverage diameter of these particles is typically less than 500 nm and,in certain embodiments, preferably less than 300 nm. The thickness ofthe conductive region 12 may also be less than 200 nm, less than 100 nm,less than 80 nm, less then 50 nm, less than 40 nm, less than 30 nm orless than 20 nm.

The size of the particles that form the conductive region 12 isdependent on the material selected for the conductive region 12 and theparticular deposition process with which the conductive region 12 isformed. The shape of the particles that form the conductive region 12may be, for example, spherical, but other structural and spatialconfigurations are possible. For instance, the particles may be slivers,cubes, ellipsoids, tubes, star-like, or take any number of other forms.In such instances, the size of a particle is defined as the smallestsphere encompassing it In certain embodiments, the particles used toform the conductive region 12 are hollow. As was previously noted, theseparticles are typically intrinsically conductive. However, the particlesmay also be formed from a polymeric material and then covered with aconductive material, such as the conductive materials described above.

Additionally, the conductive region 12 is typically formed in such a wayso as to be optically tunable. In this context, optically tunable meansthat the conductive region 12 is produced in such a way so as to have apredetermined thickness or a predetermined particle size, where thethickness and/or particle size corresponds to a known value of theprincipal wavelength of the radiation 16. For instance, evaporation,sputtering, electroless plating or electroplating may be used to controlthe thickness of the conductive region 12.

As shown in FIG. 1, the second surface 15 of the conductive region 12 isoperatively coupled with the support layer 18. For the apparatus 10, thesecond surface 15 of the conductive region 12 is deposited directly onthe support layer 18. As an alternative, at least one adhesion layer(not shown) may be present between the support layer 18 and the secondsurface 15 of the conductive region 12. Such an adhesion layer mayimprove the stability of the conductive region 12. The adhesion layer(s)may be formed using, without limitation, a layer of self-assemblingmolecules, such as silane-based molecules or thiol-based molecules. SeeFIG. 26: example of thiol SAM. Furthermore, the adhesion layer(s) mayalso comprise a layer of organic linker molecules (e.g. an adhesive). Itis noted that the adhesion layer may, but does not necessarily, have aneffect on the absorption/transmittance characteristics of the conductiveregion 12. In certain situations it is preferable that such adhesionlayers are non-metallic (i.e., non-conductive).

As was previously discussed, the recognition layer 13 is operativelycoupled with the first surface 14 of the conductive region 12. Therecognition layer 13 is a layer that includes, at a minimum,recognition, or receptor, molecules. The receptor molecules act as onepart of a binding pair, while the analyte acts as the complementarycomponent of the binding pair. Such binding pairs include, but are notlimited to, antigen/antibody, enzyme/substrate, metal/chelator,bacteria/receptor, virus/receptor, hormone/receptor, hormone/antibody,antibiotic/antibody, and nucleic acid/nucleic acid (wherein the nucleicacids are independently DNA, RNA, or mixtures thereof and wherein thenucleotides may be independently modified at the base, sugar, and/orinternucleoside linkage positions, as are well known in the art) pairs.Thus, the receptor molecules of the recognition layer 13 should havehigh specificity for the analyte being assayed. As was discussed above,such an interaction may result in a change of the dielectric constant ofboth the conductive region 12 and the recognition layer 13 andtherefore, the substrate 11.

Alternatively, the recognition layer 13 may be a layer of cells that aredeposited directly on the conductive region 12 or on an intermediatelayer, such as an adhesion layer. For the exemplary embodiment shown inFIG. 1, such an intermediate layer should be understood as being formedon the first surface 14 of the conductive region 12. The recognitionlayer 13 may be designed such that non-specific binding or adsorption issubstantially avoided.

The intermediate layer may be a self-assembling monolayer and can beformed on the conductive region 12 before binding with the first surface14 of the substrate 11. For a conductive region 12 comprising anintermediate layer, the conductive region 12 may be bound directly tothe first surface 14 of the substrate 11. Alternatively, the firstsurface 14 of the substrate 11 may be coated with chemical moleculesthat react (covalently or by physical adsorption) with the intermediatelayer

The recognition layer 13 may be formed using a self-assembled monolayer(SAM), on which the molecules of the recognition layer 13 can be bound(covalent or physical adsorption). Such a SAM, with a thickness of 0.5to 3 nm, may include at least two functional groups. The firstfunctional group may be selected such that it operatively couples withthe first surface 14 of the conductive region 12. The second functionalgroup may, as was noted above, be selected such that it interactsspecifically with the analyte being assayed. The second functional groupmay also be selected such that it reacts with a recognition molecule. Aninteraction between the recognition molecules and the analyte may, butdoes not necessarily have to, result in a change of the absorptionspectrum of the substrate 11. In certain embodiments, a SAM may beformed on the conductive region 12 before binding the conductive region12 to the first surface 14 of the substrate 11.

Method for Assaying an Analyte

The apparatus 10, shown in FIG. 1, can be employed to implement a methodto assay an analyte in a sample, as has been previously discussed. Theanalyte may be any one of various types of molecules such as, but notlimited to, biomolecules, chemical ions, and other types of cells. Morespecifically, and again without limitation, such biomolecules include,but are not limited to, hormones, proteins (such as antibodies),antigens, steroids, nucleic acids, drug metabolites or micro-organisms.The sample can also be a “blank” sample, such as a liquid sample withoutanalyte, or the substrate may be analyzed without being subjected to asample, which may be useful for correlation or determining a referencespectrum, for example.

Such a method for assaying an analyte (still with reference to FIG. 1)may include subjecting the substrate 11 to a sample to be analyzed forthe presence and/or concentration of the analyte. The substrate 11 isthen subjected to the radiation 16 produced by the radiation source 19such that the radiation 16 impinges on the substrate 11 and, inparticular, on the conductive region 12 and the recognition layer 13.The transmittance, or the absorbance, is then determined from theradiation 17 that is communicated through the substrate 11.

Determining the transmittance, or absorbance, is typically done using apredetermined wavelength (e.g., the principal wavelength of theradiation 16). While such a measurement could be made by measuring achange in peak in the spectrum of radiation 17 over time, thetransmission or absorption may also be determined by any change of theintegrated area under the curve plotting the peak spectrum or,alternatively, by simply measuring a shift in the spectrum. Forinstance, the measured transmittance or absorbance gives an indicationof the presence of an analyte in the sample because, when an analyte ispresent, the absorbance can increase or can decrease causing thespectrum to change. It will be appreciated that such spectrum changesdepend on the specific materials used for the conductive region 12, therecognition layer 13, the support layer 18 and any adhesion(intermediate or linking) layers, as well as the interaction of thesedifferent layers, and on the particular analyte being assayed.

The transmittance, or absorbance, can be measured by a conventionalabsorptiometer (also referred to as a colorimeter) or spectrometer. Themeasurement of absorbance, or transmittance, is advantageous compared tofluorescence measurements because labeling of the analyte is notrequired. Consequently, this method is less complicated to conduct ascompared to using current approaches. Furthermore, the use of aconventional light source, such as an LED, and the use of a conventionalabsorbtiometer may result in a lower cost process compared to currentapproaches. Additionally, the method may be performed in air, as “drymeasurements” or, alternatively, in a solution (e.g., water based), suchthat a flow system can be used, which may be more versatile than currentapproaches.

Conventional ELISA Assay

FIG. 2 is a schematic representation of a conventional ELISA assay. Themethods and apparatus described herein provide several advantages oversuch an approach, as will be discussed further below.

For this technique, as shown in FIG. 2, an antibody 21 is immobilized ona microtiterplate 23. However, the antibody 21 cannot be detected byconventional UV-Vis measurements, as may be done with the methodsdescribed herein. For the approach illustrated in FIG. 2, the detectionlimit of UV-Vis measurements is not adequate to detect a thin layer, ormonolayer of proteins. In a next step, the analyte, or antigen 24 isrecognized by the antibody 21. This event is also not visible by UV-Vismeasurements, for similar reasons. Therefore, a secondary antibody 25with a label, such as horseradish peroxidase (“HRP”) is used to coupleto the other side of the antigen 21, an event that is also not visibleusing UV-Vis measurements. In a next step an additional substance isadded, which is converted by the HRP label to a color in a solution 26.The color is, or can give, a quantitative estimation of the amount ofantigen in the sample. From this sequence of steps, dilution curves maybe produced. Such dilutions curves typically require extensivedevelopment and complex calculations to be accurate. Therefore, such anapproach is time consuming and costly.

In comparison, in a method such as those described herein, a thin layerof gold or nano-particles, for example, can be deposited on the bottomof a microtiterplate. The thin layer could be, in certain embodiments,gold nanoparticles. The thin layer will be referred to as gold particleshereinafter for the sake of simplicity, though it will be understoodthat particles of other metals (or alloys) can also be used. In a nextstep, the antibody is coupled to the gold particles. The absorbance ofthe gold particles coupled with the antibody is then measured. Thismeasurement results in a background absorption spectrum. In a next step,the antibody is subjected to an external medium (e.g., a sample)containing an antigen to be detected. The antigen then interacts withthe antibody, resulting in a change in the absorption spectrum. Thechange can be an increase in intensity, or a shift of the spectrum tolower/higher wavelengths. Consequently, the methods as described hereinmakes the assay process faster, simpler, less expensive, and morereliable.

Experimental Results

Protein Binding on Various Nanoparticle Films

In an experiment conducted using a method such as the one describedabove, ultra-thin gold films were prepared via evaporation and/or goldplating on mercaptosilanized glass or quartz substrates. The substrateswere cleaned by placing them in 2 M NaOH for 2 hours, followed by a 7min treatment with a 1:1:5 mixture of respectively H₂O₂ (30%), NH₄OH(25%) and ultra-pure H₂O at 80 to 90° C. in order to achieve a uniform,clean oxide layer.

3-Mercaptopropylmethyltrimethoxysilane was then dissolved in a 95:5(v/v) solvent mixture at 2%. Then, self-assembled mercaptosilaneadhesion layers were formed by immersing the substrates in this solutionfor up to 72 h. Following immersion, the substrates were removed fromthe solution and rinsed with methanol, blow-dried with N₂ and heated for10 min at 110° C. The coated substrates were then stored in N₂ untilgold evaporation or plating was performed.

For the preparation of the gold films, two techniques were used: (i)gold evaporation was performed at a speed of <5 Å/sec with an AlcatelSMC601. The final thickness on the mercaptosilanized substrates variedbetween 2 nm and 15 nm (average thickness) and (ii) electroless goldplating was performed as is described in Jin et al. (Jin, Y.; Kang, X.;Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73 (13),2843).

The mercaptosilanized substrates were then immersed overnight in acolloidal gold solution. The substrates, having a monolayer of goldnanoparticles, were subsequently immersed in an aqueous 0.4 mMhydroxylamine hydrochloride and 0.1% HAuCl₄.3H₂O solution. All glasswarewas cleaned with 2 M NaOH for 2 hours. The substrates varied in colorfrom pink, to purple, to blue. This color variation is dependent on theplating time, and therefore film thickness. After plating, thesubstrates were rinsed thoroughly with water, dried under a nitrogenstream, and were then ready for preparation for assaying.

Self-assembled-monolayers (SAMs) of 16-mercapto-1-hexadecanoic acid(“16-MHA”), 1-octadecanethiol (HS-C18) and 1-dodecanethiol (HS-C12) wereformed by immersing the clean ultra-thin gold substrates in a 1 mMthiol/ethanol solution for various periods of time. The substrates werethen rinsed with ethanol and dried under a stream of N₂.

UV-Vis spectroscopic studies were carried out using a Shimatzu UV-1601PCwith a slit width of 2 nm and a data interval of 0.5 nm. FIG. 3 isdiagram illustrating an exemplary embodiment of a slide 31 (includingone or more substrates) and a quartz cell 32 as described herein. UsingUV-Vis measurement, the absorption spectra of the ultra-thin gold-coatedslides 31 were measured in air by placing the slides 31 perpendicular toa light beam. Characterization in solution was then performed in thequartz cell 32, as shown in FIG. 3.

Surface images were then acquired in a tapping mode under ambientconditions using a PicoSPM manufactured by Molecular Imaging, USA.Silicon cantilevers having a spring constant between 1.2 and 5.5 N/mwere used at resonant frequencies between 60 and 90 kHz.

In a first experiment, Human Serum Albumin (“HSA”) was directly absorbedon the thin gold film. FIG. 4 a is graph illustrating absorbance spectrafor such a substrate with HSA directly absorbed on an evaporated thingold film. These measurements were taken at ambient conditions using aquartz substrate. FIG. 4 b is graph illustrating difference spectra forHSA directly absorbed on an isolated thin gold film and for HSA directlyabsorbed on a thin gold film on quartz. These spectra arebackground-corrected, with the background being the absorbance spectraof the thin gold film.

The deposition of HSA on 4 nm of evaporated gold was performed byplacing a drop of 1.244 mg/mL in PB for 120 min, followed by thoroughlyrinsing with pure water and drying under a stream of N₂. The next stepwas the deposition of a drop of anti-HSA 250 μg/mL in PB for 180 min,with the same rinsing and drying procedure. The absorbance changes andshifts are shown after each biosensing step. The increase in peak heightis a measure of the concentration of anti-HSA in the solution.

Self-assembled-monolayers of thiols were used to induce the absorption,or to covalently attach the bio-receptor molecules to the ultra-thingold. In a next experiment, quartz with a 4 nm film of evaporated goldwas used. The UV-Vis measurements were performed in air. The thin goldlayer was immersed for 90 min in a 10 mM 1-dodecanethiol-ethanolsolution. The absorption of HSA as a function of time was measured.Varying concentrations and times were used. The absorption was achievedusing drops having varying concentrations of HSA in HBS.

FIG. 5 a is a graph illustrating absorbance spectra ofself-assembled-monolayers of thiols on gold followed by absorption ofHSA at varying concentrations and periods of time. FIG. 5 b isdifference spectra of the spectra of FIG. 5 a. The absorbance peak shiftshown in FIG. 5 a is more pronounced in the difference spectra in FIG. 5b. The dependence on the concentration of HSA is also clearlydemonstrated by the increase in the absorbance after introducing higherconcentrations of HSA.

FIG. 6 is a graph illustrating difference spectra for variousimmunosensing applications of an example method. Immunosensingexperiments were performed on a thin layer of plated gold (8 minutes ofplating time), and demonstrate the potential of this sensing method forbiosensor applications. A self-assembled-monolayer of 16-MHA was formedon the thin gold film during a 25 min deposition period. The achievedcarboxylic terminated SAM was activated via the EDC/NHS method with amixture of 0.2 M/0.2 M EDC/NHS for 10 min. Consequently, the aminogroups of the lysine amino acids of anti-HSA (500 μg/mL in 10 mM acetatebuffer pH=5) were covalently coupled to the activated SAM surface. Thenon-reacted activated groups were blocked by rinsing for 7 min with 1 Methanolamine and the non-covalently bonded antibodies were removed byrinsing for 2 min with 10 mM glycine (HCl buffer pH=2.2). In this way, amonolayer of anti-HSA on the surface can be observed. As can be seen inFIG. 6, an enhancement in the spectra around 270 nm is visible, as is apeak shift at 600 nm. This change in the spectra can be used todetermine the concentration of anti-HSA, as has been previouslydescribed.

Nanoparticle Effects on Absorbance and Transmittance

In the case of nanoparticles, the absorption spectrum of metalnanoparticles is determined, at least in part, by (i) the particles'plasmon resonances, (ii) by bulk interband absorption. The firstmechanism, plasmon resonances, is collective oscillations of theconduction electrons on the surface of the nanoparticles. In thisrespect, the resonance frequency of a particle plasmon is determinedmainly by the dielectric properties of the metal and the surroundingmedium, respectively, and by the particle shape, i.e., the ratio of theprinciple axes. Such resonances may result in narrow,spectrally-selective absorption and an enhancement of the local lightfield confined on, and close to the surface of the metal particle. Thus,the surrounding medium influences the plasmon frequency and theamplitude of the absorption.

The second mechanism that plays a role in light absorption by smallmetal particles, as noted above, is photon interband absorption. Photoninterband absorption involves the promotion of an electron from theoccupied d-level state in a metal to an empty state above the Fermilevel. Therefore, the absorption is strongly determined by the jointdensity of d and s states of the conduction electrons. Strong absorptionindicates a “parallel” energy dispersion function. In this regard,different peaks in the spectrum can be assigned to different interbandabsorption peaks. Due to the large skin-depth of a few microns, thenano-particles absorb light in the whole bulk area of the particle.

By increasing the dielectric constant near the nanoparticle surface, anincrease of the density of the electromagnetic field at the particle'sposition enlarges the transition probability and, as such, theabsorption for bulk transitions. This effect is only visible when theparticle is smaller than the wavelength of the impinging radiationbecause such an object has too small a size to support any purelyinternal optical modes. The electric field operator internal to such asphere is determined by the extended modes, hence the dielectricconstant of the surroundings.

As the dielectric constant increases, an increase in absorption isexpected, as has been experimentally verified. However, thisrelationship may be different for particle plasmons, as the dielectricconstant of the surroundings also has a strong influence on the “wavenumber” and strength of the collective and evanescent mode ofexcitations. By coating the particles with a different material, both ashift in frequency and absorption probability is seen.

The change in dielectric constant is largest at the molecular resonance.The effect of molecular recognition between the analyte molecule and thereceptor molecule is translated in a strongly enhanced bulk absorptionsignal at this resonance. Also the strength of the analyte molecularresonance will be altered by the presence of the particle afterrecognition event. Enhanced molecular absorption at resonance leads to astrong detection signal.

Compared to SPR measurements, the methods described herein have severaladvantages. For example, these methods are typically simpler toimplement and associated apparatus can be manufactured at a relativelylow cost as compared apparatus for use with SPR measurement. Moreover,the methods and apparatus described herein allow for differingconfigurations, which are easily integrated into different biologicaltools. As yet another advantage, a normal UV-Vis spectrometer may beused for the spectral measurements, which may also provide a costsavings. In still yet another advantage, the intensity of the incomingradiation doesn't have to be focused. Therefore, relatively inexpensiveradiation sources may be used, which may provide still further costsavings as compared with prior art techniques. Of course, a laser (orother focused source) may be used as a radiation source, but is notnecessary.

FIG. 7 illustrates different absorption bands for a substrate in asample solution where particles are both in solution and covalentlyimmobilized on the substrate. The particles in solution and on thesurface both show intraband absorption bands, as designated in FIG. 7,while particles on the substrate surface also show an additional plasmoncoupling absorption band, as is also designated in FIG. 7. The differentabsorption bands display different characteristics, which are discussedbelow.

The plasmon absorption band is generally the most intense band of thosestudied. The origin and the position of this band can be explained bythe dynamics of the conduction electrons in nanoparticle(s) of asubstrate, such as described above. For particles much smaller than thewavelength(s) of the incident radiation, the electrons in the particlesmove in phase, e.g., the electrons may be considered to generate adipole under the influence of the incident radiation. Electron motionleads to the generation of surface polarization changes on each side ofthe particle, which act as a restoring force in the conduction electronsleading to a resonance frequency in the absorption spectrum. Theresultant sharp band is called a surface plasmon absorption band. Forlarger metal particles, the electrons become “dephased” and therestoring force becomes weaker. Consequentially, the electrons will be“freer” and the absorption band will broaden substantially.

In addition to particle plasmon resonances, interband absorption bandsare also observed. These interband absorption bands are generally lessintense than the plasmon absorption band and are situated at shorterwavelengths as compared to the main plasmon absorption band. This isdue, in part, to the fact that plasmon absorption bands are the resultof collective oscillations of the conduction electrons on the surface ofthe small (nano) particle. The resonance frequency of this particleplasmon is determined mainly by the dielectric functions of the metal,the surrounding medium and by the particle shape, e.g., the ratio of theprincipal axes. These resonances lead to a narrow spectrally selectiveabsorption and to an enhancement of the local light (radiation) fieldconfined on, and close to the surface of the metal particle. Thesurrounding medium influences both the plasmon frequency and theamplitude of the absorption.

The interband absorption on the other hand involves the promotion of anelectron from an occupied d-level state in the noble metal to an emptystate above the Fermi level. This absorption is strongly dependent onthe joint density of d and s states of the conduction electrons and,therefore, occurs at shorter wavelengths. Due to a large skin-depth,nanoparticles absorb light in substantially the entire bulk area of suchparticles. By increasing the dielectric constant near the nanoparticlesurface, for example, by biomolecular adsorption, an increase of thedensity of the electromagnetic field at the particles position(basically a Lorentz local field correction) increases the transitionprobability and, as such, the absorption for bulk transitions.

This effect is only visible when the subject particle is smaller thanthe wavelength of the impinging radiation because such an object has toosmall of a lateral extent to support a purely internal optical mode. Theelectric field operator internal to such a sphere is determined by theextended modes and, hence, by the dielectric constant of itssurroundings. At molecular resonances, a strong enhancement ofabsorption is observed. As the dielectric constant increases, anincreased absorption is expected and experimentally verified. This maybe different for particle plasmons because the dielectric constant oftheir surroundings also has a strong influence on the wave-number and onthe strength of the collective and evanescent modes of excitation. Ifthe particles are coated with a different material, both a shift infrequency and in absorption should be observed.

Plasmon coupling bands are generally situated at longer wavelengths andare typically broader than the plasmon absorption bands. If theinter-particle distances are on the order of the nanoparticle sizes orsmaller, the surface plasmons may interact due to dipole-dipole couplingand give rise to the plasmon coupling bands. These bands can shift theplasmon bands to longer wavelengths. In specific cases, longitudinal (L)and transverse (T) plasmon-polarization modes of gold nanoparticles on asubstrate surface can exhibit an energy split between the L and T modesand give rise to the additional plasmon coupling band.

Particle Enhanced Fluorescence

While mutual interaction between a metal nanoparticle and an organicbio-molecule enhances absorption in the particle, the absorption of theanalyte bio-molecule and related fluorescent and/or phosphorescentbehavior is also influenced by the particle. For instance, a proteinabsorption line at a certain wavelength (e.g., about 280 nm) is relatedto the fluorescence of a free bio-organic molecule. This transition isdominated by symmetry considerations which are violated when in contactwith a metal particle. Due to the disturbance of the wave functions andtheir symmetry, transitions in the molecule will be altered. Forbiddentransitions will occur and an increase of absorption and fluorescencebehavior will result. The size and form of the particle, and thedistance between the particle and the molecule are thus importantparameters that can be altered to obtain a desirable signal quality.

To take advantage of this situation, functionalized metal particles maybe used to amplify the fluorescent behavior of a specific detectorbio-molecule and, therefore, act as a sensing element. In our inventionwe use the increased absorption probability at about 280 nm (plus orminus 50 nm) of the bio-molecule next to the increased absorption at thesame wavelength of the metal particle. Therefore, in the methodsdescribed herein, the radiation spectrum that relates to such absorptionby the analyte-particle combination may be used to indicate the presenceand/or concentration of an analyte.

Another implementation of the same detection principle can be achievedby measuring the fluorescent signal itself when particles are in aliquid or attached to a surface. By generating an affinity reaction, thefluorescence/phosphorescence of the bio-molecule in the analyte isamplified. The increased fluorescent or phosphorescent light intensityis a measure of the number of detection affinity events taking place inthe liquid, or at the surface of the substrate. For such an approach, awavelength selective scheme can be used to increase the signal to noiseratio. Because it is known that absorption andfluorescence/phosphorescence can happen at different wavelengths, theuse of filters and excitation lines may be used to separate the incomingand outgoing signals, which may also result in an improved signal tonoise ratio.

Coupled Transitions

In embodiments such as those described above, the plasmon or particleresonance has an influence on the 280 nm absorption line and thelifetime of the excited states in the particle or the molecule and viceversa. This influence allows one to make phase sensitive absorption andfluorescence measurements that may also be used to improve the signal tonoise ratio. Such an approach includes exciting the molecule in a phasemodulated fashion, such that the signal of the particle coming from theparticle (plasmon) resonance will be modulated at the same frequency,allowing for a better signal to noise ratio and an increasedsensitivity. In substantially similar fashion, phase modulatedexcitation of the particle resonance will influence the 280 nmabsorption and, as such, allow for phase sensitive detection of thisline.

Method for the Fabrication of a Substrate

In one embodiment, a substrate comprising a conductive region and arecognition layer, as were described above, may be formed, for example,using the techniques illustrated in FIG. 8. Of course, other approachesare possible and the techniques shown in FIG. 8 are merely exemplary.

As is illustrated in section “1” of FIG. 8, a transparent substrate 81is provided. As is also shown in section 1 of FIG. 8, an intermediatelayer 82 is deposited on the substrate 81. Such an intermediate layer 82(which may be formed as a self-assembled monolayer, for example) may betermed an “adhesion” layer or “linking” layer, as has been previouslydiscussed.

In section “2” of FIG. 8, in-situ synthesis of (pre-activated) thiols onnanoparticles 83 is illustrated. The nanoparticles 83, in section “3” ofFIG. 8, react (e.g., bind or adhere) with the intermediate layer 82 onthe transparent substrate 81. The pre-activated thiols 83 may directlyreact with bioreceptors (an analyte) to bind the analyte for detectionand/or determination of analyte concentration, as has been previouslydescribed. Alternatively, the pre-activated thiols 83 may react (e.g.,adhere) directly with the substrate 81 without the use of theintermediate layer 82. Further, it will be appreciated, that in certainembodiments, the substrate 81 includes a conductive region, as has beenpreviously described.

Alternative Substrate

FIG. 9 illustrates an alternative substrate 90. The substrate 90 may bemanufactured by forming a plurality of holes 91 in a continuous film.The holes 91 provide for making the substrate 90 transparent toradiation used for detection of an analyte and/or determining theconcentration of the analyte, as described herein. The substrate 90 mayalso be viewed as particles that are in contact with each other whilethe spacing between the particles is constructed of circles, e.g., theholes 91.

Films used to form the substrate 90 take the form of materials thatdemonstrate predetermined absorption spectra. After binding functionalself-assembled monolayers, bioreceptors and analyte with the substrate90, the absorption spectrum is changed due to the change in theeffective dielectric constant at the metallic interface as a result ofmolecular absorption. This allows for quantitative sensing ofbioreceptor-analyte interactions. Films used for forming the substrate90 may be semi-transparent films that are formed from various materialssuch as metals, semi-conductive materials and/or alloys.

The substrate 90 may be formed using any number of techniques. Forexample, the substrate 90 may be formed using classical photolithographytechniques. Alternatively the substrate 90 may be formed usingelectron-beam techniques. Still alternatively, the substrate 90 may beformed using a polystyrene nanoparticle template method. A method ofsuch an approach is illustrated in FIG. 10.

The method of FIG. 10 includes providing a transparent substrate (asstep 100), where the substrate is modified with a first charge. Thesubstrate is then immersed (as step 101) in a solution that includesnanoparticles of polystyrene coated with molecules with a second charge,where the second charge is opposite to the first charge. The methodillustrated in FIG. 10 further includes performing a gold evaporationoperation (as step 102), and providing a device or apparatus forparticle removal (as step 103). Such an apparatus may be, for example,an adhesive coated tape. The method illustrated in FIG. 10 furtherincludes (as step 104) removal of the polystyrene nanoparticles usingthe device or apparatus provided in step 103.

In an alternative embodiment, a method for (i) preparing a substrate and(ii) for detection of (and/or determining the concentration of) ananalyte in a sample may be implemented as follows. A transparentsubstrate that includes a conductive region is provided. The substratehas chemical molecules deposited on at least one of its surfaces. Such asubstrate may be termed a “functionalized substrate”. The chemicalmolecules may have two end groups. A first end group binds to thesurface of the substrate, while the second end group binds to thenanoparticles of the conductive region.

The nanoparticles are covalently bound to the functionalized substrate.A nanoparticle film may be deposited via self-assembly from ananoparticle containing solution, e.g., via evaporation, epitaxialgrowth and/or electroless plating. These nanoparticles may includechemical molecules with functional groups that can bind recognitionmolecules, such as, for example antibodies. Alternatively, surfacenanoparticles can be modified with self-assembled monolayers that havefunctional groups that can bind recognition molecules.

The substrate may then be subjected to a reference buffer solution thatis known not to contain any analyte. After exposure to the referencebuffer solution, the substrate is subjected to radiation and theradiation that is absorbed by (or transmitted through) the substrate ismeasured. Once this reference measurement (reference spectrum) is made,the films (substrate) are subjected to the sample. If the specificanalyte is present in the sample, the analyte (antigen) then binds tothe antibody.

After exposure to the sample, the substrate (e.g., while in thereference buffer solution) is again exposed to the radiation source thatwas used to determine the reference spectrum. The radiation absorbed ortransmitted is again measured to determine a sample spectrum. Thedifference between the reference spectrum and the sample spectrum (e.g.,in the UV, VIS and near-IR region thus intraband, plasmon and plasmoncoupling absorption bands) provides both qualitative and quantitativeinformation on the concentration of the analyte in the sample.

Such a differential measurement may be performed by measuring thedifference between two substrates where a first substrate is coated witha molecule for nonspecific binding of the analyte, while the secondsubstrate is coated with recognition molecule(s) for specific bindingwith the analyte (e.g., an associated antigen) of interest. In thisapproach, there is no need for a reference buffer solution.

Sandwich Assay

As yet another alternative, a sandwich assay may be used with themethods described herein. Such a sandwich assay 110 is illustratedschematically in FIG. 11. The sandwich assay 110 includes a substrate111 that includes a conductive region and a recognition layer. Therecognition layer includes a recognition molecule 112 that can bind ananalyte 113 with a secondary antibody 114. The secondary antibody 114may contain a metal, a semiconductor material and/or an alloy. In suchapproaches, nanoparticles may also be used. This approach may furthermodify the absorption characteristics and could, therefore, enhance thesensitivity of detection and/or concentration determination.

Competitive Assays

In other example embodiments, competitive assays with antibodies orcompetitive agents containing metal, semi-conductors, alloys or othermaterials may be used to determine the presence and/or concentration ofan analyte. One example of the use of such an indirect competitiveassay, the so called “competition assay”, is illustrated in FIGS. 12 and25. As is shown in FIG. 12, a recognition molecule 121, such as anantibody, is deposited onto a conductive region of a substrate 120.Next, a sample is prepared, where the sample includes a gold-labeledanalyte 122 at a known concentration and a non-labeled analyte 123 at anunknown concentration. The substrate 120 (with the recognitionmolecule(s) 121) is subjected to the sample, where binding with thegold-labeled analyte 122 and the non-labeled analyte 123 occurs. Using acompetitive curve, the concentration of the unknown analyte may bedetermined based an analysis of the substrate 120 using previouslydescribed techniques.

Optical Fiber Apparatus

Another apparatus 130 that may used to implement methods for detectingand/or determining the concentration of an analyte (or analytes) isillustrated in FIG. 13. The apparatus 130 includes a fiber substrate131. Such substrates may have at least one optical fiber coupled with asurface of the substrate. The apparatus 130 shown in FIG. 13 includes anarray of optical fibers 132 coupled with a surface of the substrate 131.

The optical fibers 132 may be coated with a “conductive region”. Theconductive region may be implemented in like fashion as has beendescribed above. The conductive region is also operatively associatedwith a biological recognition layer, as has been previously discussed.For example, the recognition layer may be constructed of aself-assembled monolayer with immobilized antibodies. Alternatively, DNAstrengths, enzymes, ion-selective molecules, or any number of otherimmobilized receptor molecules may be used.

For the apparatus 130, the array of optical fibers 132 coupled with thesurface of the substrate 131 is compatible with a microtitre plate 133.The microtitre plate 133 may be a standard microtitre plate. Further forthe apparatus 130, each optical fiber 132 may be coated with the same ordifferent conductive region material. Likewise, the conductive region ofeach optical fiber 132 may be operatively associated with the same, orwith different recognition layer molecules.

In such an approach, each well of the microtitre plate 133 may containthe same sample, or may contain different samples. By placing differentsamples in each of the wells of the microtitre plate 133, the apparatus130 may be used for multiplex analysis of the different samples (or maybe used for multiplex analysis of the same sample in each well using thesame or different recognition molecules). In such multiplex analysisapproaches, the analysis may be further varied by each optical fiber 132containing a different recognition layer (e.g., different bioreceptors)and each well of the microtitre plate containing a different sample. Itwill be appreciated that any number of combinations of samples and/orrecognition molecules may be used.

For the apparatus 130, the optical fibers 132 may be renewed after arecognition operation. For instance, the optical fibers are renewed bydestroying the receptor-analyte binding. This destruction of bindingsmay be achieved, for example, by washing the substrate 131 in anappropriate regeneration solution. The apparatus 130 may then be used toperform another assay (recognition operation).

The apparatus 130 may also be used in conjunction with a method thatincludes modifying the optical fibers 132 with chemical molecules (suchas Si or S-based self-assembling monolayers). The chemical molecules areselected such that they bind the “conductive region” material. Forembodiments using SAMs, such SAMs may have functional groups such asmercapto, amino, CH₃, and the like.

The optical fibers 132 may then be dipped in one or more nanoparticlesolutions to allow adsorption of the “conductive regions” onto thefibers. The nanoparticle films that are formed on the optical fibers 132may also be produced by evaporation. The evaporated material may bemetallic, semi-conductive and/or an alloy, as has been previouslydiscussed. For this embodiment, the nanoparticle films are then modifiedwith Si-based self-assembled monolayers comprising functional groups.These functional groups can bind the recognition molecules (e.g.antibodies).

The method then includes performing a reference absorption measurement(in a reference buffer solution) by measuring light at the bottom of themicrotitre plate 133 with the optical fiber 132 at the top in a well ofthe microtitre plate 133. Subsequently, the optical fiber 132 (orfibers) is (are) dipped in a well containing the sample/analyte. Afterexposure to the sample, the optical fibers 132 are then dipped in thereference buffer solution again. The change in absorptionintensity/position before and after sample/analyte binding enables thedetermination of qualitative and quantitative information regarding thetarget analyte in the sample. The optical fiber 132 may then be renewedby exposing the optical fiber 132 to an appropriate regenerationsolution.

Spectrum Changes

The apparatus and methods as described in this disclosure can be used todetermine the presence and/or concentration of an analyte in a sample inany number of ways. First, the absorption/transmission difference oflight through at least one substrate (“conductive region”+self-assembledmonolayer+recognition molecule) is measured and used to allow forquantification of a specific analyte in a sample. Next, at least onesensor is subjected to the sample. Binding of a substance (the analyte)in the sample may result in a change of absorption/transmission of lightby/through the substrate in a number of ways.

First, the wavelength attributed to a plasmon effect may shift. Forinstance, a plasmon peak is typically situated in the Vis and near-IRregion. Both an absorption wavelength increase, as well as a change inan absorption peak position can be used for sensing.

Second, a wavelength at which intraband absorption occurs may beamplified by the analyte molecular resonance line. An intrabandabsorption peak is typically situated in the UV or Vis region. Both anabsorption increase as well as a change in a peak position can be usedfor sensing.

Third, a wavelength associated with plasmon coupling may be used forsensing. In this situation, particles on the surface of a recognitionlayer may demonstrate other absorption characteristics than particles insolution. For example, some particles on the surface may show asecondary peak, which may be due to plasmon coupling, as is shown inFIG. 7. This absorption peak is typically situated in the Vis to near-IRto IR region and is normally situated at longer wavelengths than theplasmon peak. This absorption peak can be used for sensingbioreceptor-analyte interactions. Both the absorption increase and achange in the peak position can be used for sensing.

As an alternative to using individual spectrum characteristics, theUV-VIS-near IR-IR spectrum as a whole may be used as a “fingerprint” forsensing analyte specific interactions. It will be appreciated that anycombination of these techniques and characteristics, as well as othersimilar techniques and characteristics may be used for analyzing analytespecific interactions.

Distance Between the Recognition Molecule and the Conductive Region

When the substrate is subjected to a sample containing the analyte, aninteraction occurs (if the analyte is present) between the analyte andthe recognition molecule (being part of the recognition layer). Aftersuch an interaction, radiation is directed through the substrate. Thetransmission of radiation through the conductive region (such as ananoparticle or discontinuous film) induces a localized surface plasmonresonance wave (resulting in the effects as described above) at thesurface of the nanoparticles. However, the length of this surfaceplasmon resonance wave decays exponentially away of the conductiveregion. The extension of the localized surface plasmon resonance wavedepends further on the nanoparticle properties (e.g., size andmaterial). The surface plasmon resonance wave extends a maximum 80 nmand often less than 60 nm or 50 nm.

It is therefore very important that the binding event (interactionbetween the analyte and the recognition molecule) under investigationhappens at a distance less than 60 nm from the nanoparticle surface, socloser than 60 nm from the nanoparticle surface, or even more preferablycloser than 50 nm from the nanoparticle surface. This means that thedistance between the conductive layer and the place of the moleculewhere the binding takes place is less than 60 nm or even better lessthan 50 nm. In case the substrate is a conducting substrate, thedistance between this surface and the active region of the recognitionmolecules in the recognition layer is less than 60 nm or more preferablyless than 50 nm. In addition, sensing closer to the conductive regiongives rise to more sensitive measurements because the localized surfaceplasmon resonance wave and thus the sensitivity is decayingexponentially away from the surface. The setup of a typical experimentfor detecting antibody/antigen interactions is represented in FIG. 14.

The sensing step of main interest (namely the antibody-antigeninteraction or other affinity interactions) happens at a certaindistance away from the surface of the nanoparticle film. Typical valuesof the different lengths important in this setup are:

-   -   Self-assembled monolayer of thiols or disulfide molecules: ˜2 nm    -   This can be combined with binding to        -   Conventional antibody: ˜15 nm        -   F(ab′)₂ ˜3-8 nm        -   Fab′ ˜3-5 nm        -   ScFv ˜2-5 nm3        -   VHH ˜2-5 nm            The antibody-antigen interaction of interest occurs at least            17 nm away from the surface of the nanoparticle film. The            sensitivity of the method of the present invention is            increased by applying smaller bioreceptors as recognition            molecule. This invention allows biosensing in a more            sensitive way. Smaller biomolecules can be obtained in            several ways.

In a first embodiment, conventional antibodies (of mouse, rat, goat,sheep), are cut into pieces. This step consists of different substeps.First, one subjects the antibodies to proteases (e.g., pepsine). The Fcpart of the antibody will be removed (FIG. 15). This Fc part is notnecessary to bind the antigen. An F(ab′)₂ fragment (˜3-8 nm) will begenerated which presents two binding sites to bind the antigen. This isa smaller, fully active, fragment (Molecular Weight of ˜110,000 dalton)compared to the full length antibody (Molecular Weight of ˜150,000dalton). This F(ab′)₂ fragment can further be chemically reduced viadifferent chemical reduction agents, e.g., 2-mercaptoethylamine (2-MEA)(FIG. 16). This subsequent modification generates small Fab′ fragments(˜3-5 nm) with a molecular weight of ˜55 000 Dalton.

Via the use of other enzymes and other reducing agents, F(ab)₂ fragmentsand Fab fragments can be generated. These fragments require a differentcoupling strategy to the self-assembled monolayers but the sensingefficiency will be similar. Applying these smaller fragments incombination with the method as described in the present invention candrastically increase the sensitivity of this invention.

In a second embodiment, single chain Fv fragments (ScFv) (2-5 nm) aregenerated. These fragments are produced via recombinant techniquesstarting from conventional full length antibodies. Details of theproduction, advantages, and disadvantages are known by those of ordinaryskill in the art. These single chain Fv fragments are also fully activefragments which can bind the antigen. In addition, they are even smallerin size. These fragments have typically a molecular weight of 25 000Dalton.

In a third embodiment, recombinant camel antibody fragments aregenerated and applied in combination with the current invention. Thesefragments are produced via recombinant techniques starting from ‘heavychain antibodies’, which are presented in a limited number of differentfamilies of animals. Details of the production, advantages, anddisadvantages are known by those of ordinary skill in the art. These VHHfragments are also fully active fragments which can bind the antigen andare even smaller in size. They have typically a molecular weight of 15000 Dalton (2-5 nm) and are therefore the smallest fully activefragments that can bind antigens (FIG. 17). These fragments aretherefore ideal to perform sensitive biosensing experiments with thetechnology described in this invention.

In a fourth embodiment, a small molecule (hormone, peptide, antibiotic,etc.), typically smaller then 3 nm, or even below 1 nm is bound on thenanoparticle surface (FIG. 22). This approach is often used in indirectcompetitive assay formats such as an inhibition assay (FIG. 25). Thislatter is often used for the detection of low molecular weight compounds(LMW) such as hormones, antibiotics, pesticides, etc. In such aninhibition assay for LMW compounds an analyte analogue is immobilized onthe sensor and the free analyte (in solution) competes with the analyteanalogue immobilized on the sensor surface for binding sites on theantibody in solution. In this type of assay, the transducer candifferentiate the relative amount of antibody binding sites occupied bythe analyte in the sample. This results in a sensor signal that isinversely proportional to the analyte concentration.

In a typical inhibition experiment, one first has to determine themaximum antibody loading onto the surface. Subsequently the same amountof the antibody is mixed with the analyte and flowed over the surface.The inhibited binding of the antibody with the analyte in solution andon the surface is a measure of the concentration of the sample afterdetermination of the maximum binding signal in the first step.

A variant on the inhibition assay is the replacement assay. In areplacement assay an antibody or antibody fragment is bound to thissurface-immobilized analyte analogue. Next, this antibody-modifiedsurface is subjected to the sample containing the analyte. The analytein the sample will go into competition with the antibodies bound ontothe surface and a decrease of the signal will be observed which canquantitatively be interpreted.

The advantage of these approaches on the TPB technology compared tonormal antibody-antigen assays relies again on the distance of thebinding event and the nanoparticle substrate. As mentioned before, theelectromagnetic field strength of the nanoparticles decays with thedistance from the surface and is dependent on the size of the particles(Zeman et al., 1987). This implies that the sensitivity will decreasewith the distance from the surface of the particles. Studies with 15 nmparticles showed that the penetration depth of the electromagnetic fieldextends 20-50 nm from the nanoparticle surface (Okamoto et al., 2000),while Nath et al. (2004) could demonstrate that particles of 39 nm coulddetect refractive index changes at distances greater than 40 nm. Forassay formats with a thiol layer (2-3 nm) and an antibody (˜15 nm)immobilized on the particles, the binding event will happen at ˜18 nmdistance from the surface. In the inhibition assay and replacement assayno antibody but a LMW compound (ampicillin) is immobilized onto the goldnanoparticles, the binding event will thus take place closer to thesurface (4-5 nm) which overcomes the drawback of decaying sensitivityaway from the surface.

Experiment

This experiment shows how VHH fragments of camel antibodies can giverise to an enhanced sensing performance. The schematic setup of theexperiment is shown in FIG. 18. Quartz slides were first cleaned for 10min with a piranha solution (1/3 H₂O₂/H₂SO₄), rinsed with water andfunctionalized with 3-mercaptopropyltrimethoxysilane (10% in 95/5ethanol/water) for 3 h. Next, the samples were rinsed with ethanol anddried in an oven for 10 min at 108° C.

A film with 50 nm gold nanoparticles was produced by self-assembly froma nanoparticle containing solution. The gold nanoparticles (50 nm) wereproduced via the procedure described by Frens (Frens, G. Nat. Phys. Sci.1973, 241, 20). The concentration of gold nanoparticles was increased 10times by centrifugation at 5000 g for more than 30 minutes. Thenanoparticles were bound on the mercapto-silane functionalized quartzslides by incubation overnight. This enabled a covalent binding betweenthe thiol-groups on the functionalized quartz slides and the goldsurface of the gold nanoparticles. Afterwards, the slides were rinsedwith DI water and dried under a stream of nitrogen gas.

Subsequently, self-assembled monolayers of thiols were deposited ontothis nanoparticle film. The slides were functionalized with thiolmolecules to allow the covalent binding of the different bioreceptors orbioreceptor fragments. Therefore, the slides were immersed in 5/95 (v/v)2-(2-(2-(2-(2-(2-(11-mercaptoundecyloxy)ethoxy)ethoxy)ethoxy)ethoxy)-ethoxy)ethoxyacetic acid and2-(2-(2-(11-mercaptoundecyloxy)ethoxy)ethoxy) ethanol in ethanol for 1hour. The resulting surface consisted of a gold nanoparticle surfacefunctionalized with both carboxylic groups to allow the coupling withthe bioreceptors or bioreceptor fragments and poly(ethylene-oxide)groups to avoid non-specific adsorption (N. Geukens, F. Frederix, G.Reekmans, E. Lammertyn, L. van Mellaert, W. Dehaen, G. Maes, J. Anné,‘In vitro analysis of type I signal peptidase affinity and specificityfor preprotein substrates by surface plasmon resonance’, Biochemical andBiophysical Research Communications 2004, 314, 459). This is illustratedin FIG. 26.

Bioreceptors were coupled using EDC/NHS method (F. Frederix, K. Bonroy,W. Laureyn, G. Reekmans, A. Campitelli, W. Dehaen and G. Maes, ‘Enhancedperformance of a biological recognition layer based on mixedself-assembled monolayers of thiols on gold’, Langmuir 2003, 19(10),4351). Both camel antibody fragments as conventional monoclonal mouseantibodies were applied. The specifications of both types ofbioreceptors are described in D. Saerens et al (D. Saerens, F. Frederix,G. Reekmans, K. Conrath, K. Jans, L. Brys, L. Huang, E. Bosmans, G.Maes, G. Borghs, S. Muyldermans, ‘Engineering camel single-domainantibodies and interphasechemistry for human prostate-specific antigenbiosensing’, Analytical Chemistry 2005, 77(23), 7547) and Huang et al(L. Huang, D. Saerens, G. Reekmans, J.-M. Friedt, F. Frederix, L.Francis, S. Muyldermans, A. Campitelli, ‘The Combination of mixedSelf-Assembled Monolayers and VHH camel antibodies for Enhanced PSAImmunosensing’, Biosensors and Bioelectronics 2005, 21, 483).

The actual localized surface plasmon resonance or transmission plasmonbiosensor experiments were performed using differential measurementswith a Shimadzu UV-1601PC spectrophotometer with a slit width of 2 nmand a data interval of 0.5 nm. The aim is to detect the prostatespecific antigen (PSA, SCIPAC) at different concentration in HBS (10 mM4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid, 150 mM NaCl, 3.4 mMethylenediaminetetraacetate, and 0.005% Tween 20 (pH 7.4)). Differentialmeasurements implies that the difference UV-Vis absorption spectrum ismeasured between an antibody coated functionalized quartz slide andslide without an antibody upon contact with the PSA containing sample.Differential measurements allow for more sensitive measurements due tothe compensation of experimental errors, temperature fluctuations andother instrument related fluctuations. The time frame between differentinjections of PSA varies between 35 and 40 min as indicated in FIG. 19.

The camel antibody fragments applied in this study have a molecularweight of 15,000 Daltons while the conventional mouse antibodies havetypically a molecular weight of 150,000 Daltons. The camel antibodyfragments are therefore almost 10 times smaller than conventionalantibodies. The camel antibody fragments applied in this study have onlyone recognition side/region while conventional mouse antibodies have tworecognition binding sites. It is therefore expected that the use ofcamel antibody fragments enables an increase in the amount of bindingsites on the surface of a factor 5 (10/2). However, if one compares thePSA recognition signals for both camel antibody fragments and normalantibodies, the signal on camel antibody fragments exceeds the factor of5. This indicates that other factors play a major role. Due to thesmaller size of the camel antibody fragments, the recognition event orthe PSA binding event happens closer to the surface due to the smallersize of camel antibody fragments compared to conventional mouse antibodyfragments. The additional enhancement effect is therefore contributed toby the increased sensitivity of localized surface plasmon resonance uponthe use of smaller receptor fragments, which is a advantageouscharacteristic of this invention. This can be explained by the fact thatthe sensitivity of this technique decays away from the surface. Applyingsmaller binding fragments allows monitoring of the binding event at aposition were the sensitivity is higher.

Experiment

Another approach to further increase the sensitivity of the transmissionplasmon biosensor technique or the localized surface plasmon biosensortechnique is the use of a sandwich assay using colloidal goldfunctionalized secondary antibodies. An illustration of this approach isshown in FIG. 20

To illustrate the ability of this approach, an anti-PSA antibody shouldbe coupled to the gold nanoparticles. In this specific assay, theanti-PSA antibodies were coupled to commercial 20 nm (supplied by BBI)gold nanoparticles. The anti-PSA antibodies were first biotinylatedaccording to the manufacturers' instructions. Next, 1 mL of BBInanoparticles were mixed with 250 μg of biotinylated anti-PSA monoclonalmouse antibodies. After mixing and centrifugation, the anti-PSA coatedgold nanoparticles were isolated and applied in the localized surfaceplasmon resonance or Transmission Plasmon Biosensor technique. Afterfunctionalization the quartz slides with nanoparticles and camelantibody fragments, 10 ng/mL of PSA in HBS was pursed over the camelantibody fragment coated gold nanoparticle slide. The signal wasmeasured and an additional enhancement effect was observed, whichexemplifies the possible sensitivity increases that can be expectedusing this approach (see FIG. 21).

In the above-mentioned examples the increase is doubled, but additionalpeaks were observed due to plasmon coupling, which could furtherincrease the sensitivity of this approach.

Experiment

The above-described replacement inhibition assay experiment wasperformed with ampicillin as an analyte/antigen and 19C9 as penicillinspecific monoclonal antibody (mAb). First, anti-ampicillin antibodieswere bound onto a nanoparticle film which was functionalized withampicillin molecules via a self-assembled monolayer (FIGS. 22 and 23).Next, this surface was subjected to the sample containing the analyte(ampicillin) in a high concentration (1 mM). The bound anti-ampicillinantibodies release themselves from the surface due to the competitionwith the ampicillin molecules in the sample. If mixtures of ampicillinand a fixed concentration of 19C9 are then sent over the surface, thesignal will be reversely related to the concentration of ampicillin. Ifa high concentration of ampicillin is present in the sample, the signaldecreases towards zero (FIG. 24).

A detailed description of the experiment is given below. Quartz slideswere first cleaned for 10 min with a piranha solution (1/3 H₂O₂/H₂SO₄).Subsequently the samples were rinsed with water and functionalized with3-mercapto-propyltrimethoxysilane (10% (v/v) in 95/5 (v/v)ethanol/water) for 3 h. Next, the samples were rinsed with ethanol anddried in an oven for 10 min at 108° C.

The gold nanoparticles (50 nm) were produced via the procedure describedby Frens (Frens, G. Nat. Phys. Sci. 1973, 241, 20). The nanoparticleswere bound on the mercapto-silane functionalized quartz slides by anovernight incubation. This enables a covalent binding between thethiol-groups on the functionalized quartz slides and the gold surface ofthe gold nanoparticles. Afterwards, the slides were rinsed with DI waterand dried under a stream of nitrogen gas.

Subsequently, the slides were functionalized with thiol molecules toallow the covalent binding of the receptor molecules (in this examplethe penicillin ampicillin). Therefore, the slides were immersed in a5/95 (v/v) solution of 1 mM2-(2-(2-(2-(2-(2-(11-mercaptoundecyloxy)ethoxy)ethoxy)ethoxy)ethoxy)-ethoxy)ethoxyaceticacid and 2-(2-(2-(11-mercaptoundecyloxy)ethoxy)ethoxy)ethanol in ethanolfor 1 hours. The resulting surface consisted of a gold nanoparticlesurface functionalized with both carboxylic groups to allow the couplingwith the receptor molecules and poly(ethylene-oxide) groups to avoidnon-specific adsorption.

Via their amino groups, the ampicillin molecules (supplied by Sigma, CASnumber 69-52-3) were coupled onto the thiol molecules using a previouslydescribed EDC/NHS method (F. Frederix, K. Bonroy, W. Laureyn, G.Reekmans, A. Campitelli, W. Dehaen and G. Maes, ‘Enhanced performance ofa biological recognition layer based on mixed self-assembled monolayersof thiols on gold’, Langmuir 2003, 19(10), 4351).

The actual localized surface plasmon resonance or transmission plasmonbiosensor experiments were performed using differential measurementswith a Shimadzu UV-1601PC spectrophotometer with a slit width of 2 nmand a data interval of 0.5 nm. The aim is to measure binding ofpenicillin-specific antibodies (monoclonal antibody clone 19C9 (CliquestP. et al. (2001, J. of Food and Agricultural Chemistry, 49, 3349) inphosphate buffer saline (PBS) (10 mM phosphate, 3 mM KCl, 137 mM NaCl,pH 7.4). The 19C9 binding can be inhibited by ampicillin moleculespresent in the sample. The inhibition of the 19C9 binding is related tothe concentration of ampicillin in the sample. Differential measurementsimplies that the difference UV-Vis absorption spectrum is measuredbetween an ampicillin coated functionalized quartz slide and slidewithout ampicillin upon contact with the 19C9 antibody containingsample. Differential measurements allow for more sensitive measurementsdue to the compensation of experimental errors, temperaturefluctuations, and other instrument related fluctuations.

The ampicillin molecules (referred to as an analyte analogue) applied inthis study have a molecular weight of ˜370 Dalton with a size of lessthan 2 nm. Due to the smaller size of the analyte analogue compared tobigger receptor molecules, the recognition event or the 19C9 bindingevent happens closer to the surface. The additional enhancement effectis therefore contributed to by the increased sensitivity of localizedsurface plasmon resonance upon the use of smaller receptor fragments,which is an advantageous characteristic of this invention. This can beexplained by the fact that the sensitivity of this technique decays awayfrom the surface.

CONCLUSION

While a number of aspects and embodiments have been discussed above, itwill be appreciated that various modifications, permutations, additionsand/or sub-combinations of these aspects and embodiments are possible.It is therefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such modifications,permutations, additions and/or sub-combinations as are within their truespirit and scope.

1. A method for determining the concentration of an analyte within asample, the method comprising: providing a first substrate comprising aconductive region and a recognition layer, the conductive region havinga first surface operatively coupled with the recognition layer, therecognition layer comprising at least one recognition molecule, whereinsaid conductive region comprises at least one particle formed of aconductive material and having a hollow tubular shape with an averagediameter of less than 300 nm and wherein the distance between the firstsurface of the conductive region and the recognition molecule isselected such that when the analyte is bound to the recognition layerthe combination of the at least one particle and the analyte exhibits aparticle plasmon effect when radiation is directed through theconductive region and the recognition layer; contacting the substratewith the sample to bind at least some of any analyte present in thesample with the recognition molecule; directing radiation through theconductive region and the recognition layer, measuring at least a partof a spectrum of radiation that is absorbed or transmitted by or throughthe substrate and manifesting a particle plasmon effect; comparing theat least part of the spectrum with a reference spectrum and determiningthe difference; and determining the concentration of the analyte fromthe difference.
 2. The method according to claim 1, wherein the distancebetween said first surface and the part of said recognition moleculewhere binding takes place is less than 60 nm.
 3. The method as recitedin claim 1 wherein the distance between said first surface and the partof said recognition molecule where binding takes place is less than 17nm.
 4. The method as recited in claim 1 wherein the distance betweensaid first surface and the part of said recognition molecule wherebinding takes place is between 4 and 17 nm.
 5. The method as recited inclaim 1 wherein the recognition molecule is subjected to enzymaticcleavage such that only the active part of the recognition molecules ispart of the recognition layer.
 6. The method as recited in claim 1wherein the recognition molecule is a small molecule that functions as arecognition element in an inhibition or replacement assay.
 7. The methodas recited in claim 1, wherein the at least one particle exhibits aparticle plasmon effect and a bulk interband absorption and a plasmoncoupling band.
 8. The method as recited in claim 1, wherein the at leastone particle is formed of metal.
 9. The method as recited in claim 1,wherein the at least one particle is an alloy of at least two metals.10. The method as recited in claim 1, wherein the conductive regioncomprises semiconductive particles and metallic particles.
 11. Themethod as recited in claim 1, wherein the conductive region comprises atleast two particles and the edge to edge distance of the at least twoparticles is between 1 nm and 5 μm.
 12. The method as recited in claim1, wherein the conductive region comprises at least two particles andthe edge to edge distance of the particles is between 1 nm and 1 μm. 13.The method as recited in claim 1, wherein the average diameter of the atleast one particle is smaller in dimension than a principal wavelengthof the radiation.
 14. The method as recited in claim 1, wherein aninteraction between the analyte and the recognition layer results in achange in a dielectric constant of the recognition layer.
 15. The methodas recited in claim 1, wherein the substrate further comprises a supportlayer and a second surface of the conductive region, the second surfacebeing operatively coupled with the support layer.
 16. The method asrecited in claim 15, wherein the support layer is optically transparentto the radiation.
 17. The method as recited in claim 15, wherein thesupport layer is optically semi-transparent to the radiation.
 18. Themethod as recited in claim 1, wherein the recognition layer comprises anintermediate layer and a recognition molecule.
 19. The method as recitedin claim 1, wherein the recognition layer comprises a self-assemblingmonolayer.
 20. The method as recited in claim 1, wherein the substratecomprises a plurality of conductive regions, the plurality of conductiveregions being arranged in an array.
 21. The method as recited in claim20, wherein the substrate is arranged as a microtitre plate.
 22. Themethod as recited in claim 1, wherein an intensity of the radiationabsorbed or transmitted by or through the substrate is determined as afunction of a wavelength of the radiation.
 23. A method for determiningthe concentration of an analyte within a sample, the method comprising:providing a first substrate comprising a conductive region and arecognition layer, the conductive region having a first surfaceoperatively coupled with the recognition layer, the recognition layercomprising at least one recognition molecule, wherein said conductiveregion comprises at least one particle formed of a conductive materialand having a hollow tubular shape with an average diameter of less than300 nm and wherein the distance between the first surface of theconductive region and the recognition molecule is selected such thatwhen the analyte is bound to the recognition layer the combination ofthe at least one particle and the analyte exhibits a particle plasmoneffect; contacting the substrate with the sample to bind at least someof any analyte present in the sample with the recognition molecule;directing radiation through the conductive region and the recognitionlayer, measuring at least a part of a spectrum of radiation that isabsorbed or transmitted by or through the substrate and manifesting aparticle plasmon effect; providing a second substrate; subjecting thesecond substrate to a reference sample; directing radiation through thesecond substrate; measuring the intensity of the radiation absorbed ortransmitted by or through the second substrate; and comparing anintensity of the radiation absorbed or transmitted by or through thesecond substrate with an intensity of the radiation absorbed ortransmitted by or through the first substrate to determine theconcentration of the analyte on the first substrate.
 24. The method ofclaim 1, wherein the conductive region comprises at least two particlesand a combination of the at least two particles and the analyte furtherexhibits a plasmon coupling effect.